1. Field of the Invention
This invention relates to a digital-to-analog converter, and in particular to a digital-to-analog converter that attempts to reduce an amount of noise present in an output signal, and to a method of operation of a digital-to-analog converter.
2. Description of the Related Art
In many electronics devices, digital signals are used in order to allow signal processing operations to be performed, or to allow data to be stored in a convenient form. However, it is often necessary to use analog signals to drive, for example, analog transducers such as speakers in audio equipment. In such situations, and many others, digital-to-analog converters are used to convert a digital input signal to an analog output signal.
Digital-to-analog converters are known, in which a value of a digital input signal is used to select a number of single-bit digital-to-analog converter elements. The outputs of these single-bit digital-to-analog converter elements are then summed together, in order to produce an analog output signal.
It is also known that the single-bit digital-to-analog converter elements can be formed in a logical ring, with the element or elements selected in any time period following on consecutively from the element or elements selected in the preceding time period. The document “A 19-Bit Low-Power Multibit Sigma-Delta ADC Based on Data Weighted Averaging,” Nys et al., IEEE Journal of Solid-State Circuits, Vol. 32, No. 7, July 1997, pp. 933-942 discloses a digital-to-analog converter of this type. This has the advantage that distortion is reduced because, for any given input value, different elements can be selected in different time periods. Moreover, all of the elements are selected on essentially equal numbers of occasions averaged over many time periods, reducing the low frequency components of noise introduced by possible mismatch between the elements.
However, there remains the issue that, if the same input signal is received consecutively, the group of selected elements will repeat. For example, in a case where there are eight single-bit digital-to-analog converter elements, if the value of the input signal is such that a group of four elements is to be selected, then a first group of four elements will be selected in a first time period, the four remaining elements will be selected in a second time period, and the first group of four elements will be selected again in a third time period, and the four remaining elements will be selected again in a fourth time period. In this case, the sequence repeats with a frequency of once per two time periods, leading to a tone at this frequency. Other input signal values lead in the same way to tones at other frequencies. Thus, where the input signal includes any DC component, as is typical, this will cause tones to appear in the output.